Bandwidth problems have long restricted the ability of cable television systems to provide information services to subscribers. Although a coaxial cable system may permit a cable system operator to provide, for example, 50 television channels, each 6 MHz wide, with a total bandwidth of 300 MHz, this total bandwidth is insufficient to permit an arrangement wherein each subscriber may have, in addition to these 50 channels, an interactive information service that functions independently of interactive information services to all other subscribers and provides full color video, motion typical of movies or television, and sound.
The reason for the insufficiency in bandwidth is apparent on a consideration of the demands on the system. Typically a subscriber on a cable system obtains information services over a communication path that starts at the headend, proceeds over one of typically a number of trunks, and then over one of a number of feeders, and then over one of a number of taps. Each feeder may have, for example, fifty or more subscribers, and each trunk might serve a hundred or more feeders. The result is that 5000 subscribers per trunk is not atypical. Thus merely to provide a private one-way information service, and nothing else, to each of these 5000 subscribers would require the trunk to carry 5000 different signals, each using about 6 MHz of bandwidth, and would alone require a trunk bandwidth of 30 GHz, which is nearly two orders of magnitude greater than provided by a typical coaxial cable system. Even if it were assumed that at most only ten percent of these subscribers would want a private one-way information service at any given time, the required trunk bandwidth for this service alone would still be nearly an order of magnitude greater than provided by a typical system. While video compression schemes may bring the bandwidth requirements within more practical limits, each subscriber would then need to be provided with his own decompression unit.
Another problem lies in how to handle the switching and computing demands on the headend to provide separate and private information service to potentially hundreds of thousands of subscribers simultaneously.